Method for producing ceramic structure

ABSTRACT

There is provided a method for producing a ceramic structure, in which a concentration of at least one kind of inflammable gas (gas to be measured) in the combustion furnace is measured upon degreasing, and a condition such as temperature rise speed is adjusted according to a total concentration of the gas to be measured to control the total concentration of the gas to be measured to be a concentration of 0 to 75% of an explosion lower limit concentration of the whole gas to be measured. The method can effectively inhibit an explosion inside the furnace, can reduce the production cost, and can produce a ceramic structure with high production efficiency.

BACKGROUND OF THE INVENTION AND RELATED ART STATEMENT

The present invention relates to a method for producing a ceramicstructure by degreasing and firing a ceramic formed body.

Since ceramic material such as silica and alumina is excellent inmechanical strength and durability in comparison with organic materialsuch as plastic, ceramic material is variously used as, for example,structure material and electric/electronic material. It is general thatthe ceramic material is used as a ceramic structure subjected tomachining to have a shape suitable for each usage. For a use such as adiesel particulate filter (DPF) for trapping particulate matter (PM)exhausted from a diesel engine (diesel engine or the like of anautomobile), there is suitably used a honeycomb-shaped ceramic structure(ceramic honeycomb structure) having a large number of cells formed andseparated from each other by partition walls.

As a method for producing such a ceramic structure, there has beendisclosed a production method in which an organic type additive such asan organic binder and a forming auxiliary is mixed with a ceramic rawmaterial (e.g. ceramic powder) and a dispersion medium (e.g. water) togive a mixture, which is kneaded to give clay, and the clay is formed byextrusion forming or the like, dried, and degreased/fired (seeJP-A-2002-219319). In a production method as described above,degreasing/firing is conducted often with a combustion furnace whichheats up using combustion heat of fuel gas as a heat source (seeJP-B-7-45348).

By the way, there has been required a ceramic honeycomb structure havinga low pressure loss and high treatability in a filter use such as a DPFin recent years, and increase in porosity and decrease in partition wallthickness of a ceramic honeycomb structure is rapidly making progress.This steadily increases an amount of an organic type additive (organicpore-forming agent for increasing porosity, or an organic binder or aforming auxiliary for improving formability and shape retainability).

In accordance with changes in such a state, a danger of explosion upondegreasing/firing, which has conventionally been unconsidered, has begunbeing pointed out. That is, since a large amount of inflammable gas isgenerated due to decomposition of a large amount of organic typeadditive contained in a ceramic formed body, and a concentration ofinflammable gas in the furnace is remarkably raised in degreasing/firingusing a combustion furnace as described above, a danger of explosion inthe furnace has arisen.

As a measure for preventing an explosion in a furnace, there can beconsidered a method in which degreasing/firing is conducted in anatmosphere having a very low oxygen content with an inert gas atmospherein a furnace. However, since this method uses an inert gas, it isnecessary to substitute inert gas for gas in the furnace in addition torise in production cost, and complex operations are forced in theprocess. These problems still remain, and the method is stillinsufficient.

In addition, there can be considered a method in which an amount ofinflammable gas generated from a ceramic formed body is suppressed bylowering temperature rise speed an amount of the ceramic formed bodyarranged in a furnace to a level where it is sufficiently safe on thebasis of a conventional rule based on experience. However, since thetemperature rise speed is determined on the basis of a conventional rulebased on experience in this method, there still arise problems that i)the method is yet insufficient from the viewpoint of effectivelypreventing explosion in a furnace, and that ii) temperature rise speedand an amount of the ceramic formed body is lowered to a level which isvery low in comparison with a practically operable level to lowerproduction efficiency, and thus there is left some room for improvement.

As described above, there has not been disclosed a method capable ofreducing the production cost and producing a ceramic structure with highproduction efficiency in addition to effectively preventing explosion ina furnace. Therefore, creation of such a production method is earnestlydesired by the industrial world.

The present invention has been made to solve the conventional technicalproblems described above and provides a method for producing a ceramicstructure, the method being capable of reducing the production cost andproducing a ceramic structure with high production efficiency inaddition to effectively preventing explosion in a furnace.

SUMMARY OF THE INVENTION

The present inventors found out that the above problems can be solved bya method in which a concentration of at least one kind of inflammablegas (hereinbelow sometimes referred to as “gas to be measured”) in thecombustion furnace is measured; and temperature rise speed or an amountof the ceramic formed body arranged in the combustion furnace isadjusted according to a total concentration of the gas to be measured tocontrol the total concentration of the gas to be measured to be apredetermined concentration or lower. Specifically, according to thepresent invention, there is provided the following method for producinga ceramic structure.

[1] A method for producing a ceramic structure by conducting degreasingand firing a ceramic formed body containing a ceramic raw material andan organic type additive in a combustion furnace in an oxygen-containingatmosphere;

wherein a concentration [C_(Gn)] of at least one kind of inflammable gas(gas to be measured) in the combustion furnace is measured upondegreasing; and at least one condition selected from the groupconsisting of temperature rise speed, an amount of air introduced into aburner, an amount of fuel gas introduced into a burner, and an amount ofthe ceramic formed body arranged in the combustion furnace is adjustedaccording to a total concentration of the gas to be measured to controlthe total concentration of the gas to be measured to be a concentrationof 0 to 75% of an explosion lower limit concentration [L₁]calculatedfrom the following formulae (1) and (2) of the whole gas to be measured.$\begin{matrix}{\lbrack {{Formula}\quad 1} \rbrack\quad{P_{Gn} = {100 \times {C_{Gn}/{\sum\limits_{n = 1}^{n}C_{Gn}}}}}} & \begin{matrix}\quad \\(1)\end{matrix} \\{\lbrack {{Formula}\quad 2} \rbrack\quad{L_{1} = {100/{\sum\limits_{n = 1}^{n}( {P_{Gn}/L_{Gn}} )}}}} & \begin{matrix}\quad \\(2)\end{matrix}\end{matrix}$(where P_(Gn): ratio of a concentration of the gas to be measured to thetotal concentration of the gas to be measured, C_(Gn): concentration(vol. %) of each gas to be measured), L₁: explosion lower limitconcentration (vol. %) of the whole gas to be measured, and L_(Gn):lower limit concentration (vol. %) of each gas to be measured)[2] A method for producing a ceramic structure according to the above[1], wherein a THC (total hydrocarbon) concentration [C_(HC)] in termsof a specific hydrocarbon and a concentration [C_(CO)] of carbonmonoxide in the combustion furnace is measured upon degreasing; and thetotal concentration [C_(HC)+C_(CO)] of THC and carbon monoxide isadjusted to be a concentration of 0 to 75% of an explosion lower limitconcentration [L₂] calculated from the following formulae (1) and (2) ofthe explosion lower limit concentration [L₂] of the whole THC and carbonmonoxide.P _(HC)=100×C _(HC)/(C _(HC) +C _(CO))  (3)P _(CO)=100×C _(CO)/(C _(HC) +C _(CO))  (4)L ₂=100/{(P _(HC) /L _(HC))+(P _(CO) /L _(CO))}  (5)(where P_(HC): ratio of a THC concentration to the total concentrationof THC and carbon monoxide, C_(HC): THC concentration (vol. %), P_(CO):ratio of carbon monoxide concentration to the total concentration of THCand carbon monoxide, C_(CO): carbon monoxide concentration (vol. %), L₂:explosion lower limit concentration (vol. %) of the whole THC and carbonmonoxide, L_(HC): explosion lower limit concentration (vol. %) of aspecific hydrocarbon; and L_(CO): explosion lower limit concentration(vol. %) of carbon monoxide)[3] A method for producing a ceramic structure according to the above[1] or [2], wherein feedback control is conducted by adjusting at leastone condition selected from the group consisting of temperature risespeed, an amount of air introduced into a burner, and an amount of fuelgas introduced into a burner according to a total concentration of thegas to be measured upon degreasing.[4] A method for producing a ceramic structure according to any one ofthe above [1] to [3], wherein the total concentration of the gas to bemeasured is controlled in a temperature range from 150 to 400° C.

A method for producing a ceramic structure according to any one of theabove [1] to [4], wherein the total concentration of the gas to bemeasured is controlled to be a concentration of 0 to 25% of theexplosion lower limit concentration [L₁]of the whole gas to be measuredin a temperature range from 150 to 400° C.

A production method of a ceramic structure of the present invention caneffectively prevent explosion in a furnace, reduce the production cost,and produce a ceramic structure with high production efficiency.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional view showing a constitution example of acombustion furnace usable in a production method of the presentinvention.

FIG. 2 is a schematic sectional view showing a constitution of acombustion furnace (single kiln) used in Examples.

DETAILED DESCRIPTION OF THE INVENTION

Hereinbelow, a best mode for carrying out a production method of aceramic structure of the present invention will be describedspecifically. However, a production method of the present inventionwidely includes all modes having items specifying the invention andshould not be limited to the following modes.

The production method of a ceramic structure of the present invention isa method in which a ceramic structure is obtained by conductingdegreasing and firing a ceramic formed body containing a ceramic rawmaterial and an organic type additive in a combustion furnace in anoxygen-containing atmosphere; and a concentration of at least one kindof inflammable gas (gas to be measured) in the combustion furnace ismeasured upon degreasing, and temperature rise speed, an amount of theceramic formed body arranged in the combustion furnace, or the like, isadjusted according to a total concentration of the gas to be measured tocontrol the total concentration of the gas to be measured to be apredetermined concentration.

[1] Ceramic Formed Body

In a production method of the present invention, a ceramic formed bodyto be fired contains a ceramic raw material and an organic typeadditive.

[1-1] Ceramic Raw Material

In a production method of the present invention, “ceramic raw material”means ceramic powdered or particle matter which can serve as anaggregate particle of a ceramic structure. There is no limitation on aceramic seed, and various ceramic materials which have conventionallybeen employed as a constituent of a ceramic structure can be used. Therecan suitably be used, for example, silica, alumina, titania, zirconia,mullite, aluminum titanate, silicon carbide, cordierite forming material(a material containing talc, kaolin, aluminum hydroxide, or the like insuch a manner that a composition after firing is a theoreticalcomposition (2MgO.2Al₂O₃.5SiO₂) of cordierite). These ceramic materialsmay be used alone or in combination in the form of powder or particlematter.

[1-2] Organic Type Additive

Generally, “an organic type additive” means organic matter added to theabove ceramic raw material as a raw material of a ceramic formed body.Examples of the organic type additive include a pore forming agent, anorganic binder, and a dispersant. However, “an organic type additive” inthe present invention includes inorganic matter as long as it decomposesitself or generates inflammable gas by combustion. An example of suchinorganic matter is carbon.

The pore forming agent is an additive for raising porosity to obtain aceramic structure with high porosity by forming pores due to burningupon firing a honeycomb dried body, and combustibles burning out in adegreasing/firing step can suitably be used. Examples of the poreforming agent include carbon such as graphite, flour, starch, phenolresin, (methyl methacrylate), polyethylene, and poly(ethyleneterephthalate). As such a pore forming agent, a resin microcapsule issometimes used in order to reduce heat or generation of thermal stressupon combustion.

An organic binder is an additive functioning as a reinforcer impartingflowability, shape-retainability, handling strength, etc., to clay orthe like serving as a raw material of a ceramic formed body, and anorganic polymer or the lime can suitably be used. Examples of theorganic binder include hydroxypropylmethyl cellulose, methyl cellulose,hydroxyethyl cellulose, carboxylmethyl cellulose, and poly(vinylalcohol).

A dispersant is an additive for facilitating dispersion of ceramic rawmaterial or the like to obtain homogeneous clay. Therefore, as thedispersant, a so-called surfactant can suitably be used. Examples of thedispersant include ethylene glycol, dextrin, fatty acid soap, andpolyalcohol.

[1-3] Other Constituents

A ceramic formed body in a production method of the present inventionmay include other substances as constituents as long as the ceramicformed body contains ceramic raw material and an organic type additive.Examples of other substances include a dispersion medium such as waterand alcohol, and metal raw material in the case that the ceramic formedbody is constituted by a composite material of ceramic and metal, forexample, a metal silicon powder in the case that the ceramic formed bodyis constituted by metal silicon-bonded silicon carbide (Si—SiC).

[1-4] Production Method of Formed Body

A ceramic formed body can be produced by mixing a dispersion medium andother additives as necessary to ceramic raw material and an organic typeadditive as essential components to obtain a mixture, which is kneadedand then formed.

Mixing them can be conducted with a conventionally known mixer, e.g., asigma kneader and a ribbon mixer. Kneading the mixture can be conductedwith a conventionally known kneader, e.g., a sigma kneader, a Bumbarymixer, a screw type extruder-kneader, a vacuum kneading machine, and abiaxial continuous kneading-extruder.

Forming can be conducted by a conventionally known forming method suchas extrusion molding, injection molding, and press molding. However, inthe case of obtaining a formed body having a honeycomb structure, it ispreferable to form by extrusion molding in which clay is extruded froman extrusion die with silts having a shape complementary to that of thepartition wall to be formed.

The formed body obtained as described above may be dried prior todegreasing/firing. There can be employed a conventionally known dryingmethod such as hot air drying, microwave drying, dielectric drying,vacuum drying and freeze drying.

[2] Degreasing Step

A production method of the present invention includes a degreasing stepfor removing an organic type additive contained in a ceramic formed bodyby combustion. Degreasing is conducted before firing or in process oftemperature rise of firing. Since the combustion temperature of theorganic binder is about 180 to 300° C., and the maximum combustiontemperature of the pore forming agent is about 400° C., the degreasingtemperature may be about 200 to 1000° C. Though the degreasing time isnot particularly limited, it is generally about 10 to 150 hours.

In a production method of the present invention, degreasing is conductedin a combustion furnace in an oxygen-containing atmosphere. Since such amethod is not required to use inert gas such as nitrogen, the productioncost can be reduced in comparison with a method in which degreasing isconducted in an atmosphere having a very low oxygen content with aninert gas atmosphere in the furnace, and the process can be simplifiedsince it is not necessary to substitute inert gas for gas inside thefurnace, and thereby production efficiency can be raised.

Meanwhile, in the case of degreasing a ceramic formed body containing alarge amount of organic type additive in such a method as in recentyears, there is a fear of explosion inside the furnace sinceconcentration of inflammable gas in the furnace remarkably rises.Therefore, in a production method of the present invention, it wasdecided to employ a method where a concentration of at least one kind ofinflammable gas (gas to be measured) in the combustion furnace ismeasured; and at least one condition selected from the group consistingof temperature rise speed, an amount of air introduced into a burner, anamount of fuel gas introduced into a burner, and an amount of theceramic formed body arranged in the combustion furnace is adjustedaccording to a total concentration of the gas to be measured to controlthe total concentration of the gas to be measured.

According to such a method, explosion in the furnace can effectively beprevented since a concentration of inflammable gas in the furnace can bedetermined based on the relation with an explosion lower limitconcentration. In addition, since it is controlled on the basis of apractically measured concentration of inflammable gas, the method hashigh reliability in comparison with a method in which temperature risespeed or an amount of a ceramic formed body arranged in the furnace isdetermined based on an experience value. Further, since temperature risespeed and an amount of a ceramic formed body arranged in the furnace isdetermined based on a practically measured concentration of inflammablegas, it is not required to operate at a very low level in comparisonwith a practically operable level in consideration of safety rate, andtherefore, improvement in production efficiency can be expected.

If a production method of the present invention is described morespecifically, a concentration of at least one kind of inflammable gas(gas to be measured) in the combustion furnace is measured upondegreasing; and at least one condition selected from the groupconsisting of temperature rise speed, an amount of air introduced into aburner, an amount of fuel gas introduced into a burner, and an amount ofthe ceramic formed body arranged in the combustion furnace is adjustedaccording to a total concentration of the gas to be measured to controlthe total concentration of the gas to be measured to be a concentrationof 0 to 75% of an explosion lower limit concentration.

Examples of inflammable gas generated in the combustion furnace uponproducing a ceramic structure include decomposed matter and a volatilecomponent. In a production method of the present invention, at least onekind of inflammable gas (gas to be measured) in the combustion furnaceis measured. There is no particular limitation on kind of gas to bemeasured as long as it is inflammable gas generated in a combustionfurnace, and arbitrary gas can be selected from inflammable gas capableof causing explosion in a furnace. Only one kind of inflammable gas maybe selected, or two or more kinds of inflammable gas may be selected forthe gas to be measured.

However, most of the inflammable gas generated in the furnace is mixedgas of various kinds of hydrocarbon-based compounds. Therefore, it ispreferable to select a hydrocarbon-based compound as the gas to bemeasured from the viewpoint of effectively preventing explosion in thefurnace. Since a total hydrocarbon concentration meter (hereinbelowreferred to as a “THC meter”, e.g., commercial name: EHF-770 produced byAnatec Yanaco?) on the market can measure concentration and can expressthe concentration by a value in terms of a specific hydrocarbon (e.g.,methane, propane, and ethylene), it can suitably be used in a productionmethod of the present invention.

In addition, inflammable gas generated in a furnace is exemplified bycarbon monoxide besides hydrocarbon-based compounds. Therefore, it ispreferable to select carbon monoxide in addition to a carbon-basedcompound as the gas to be measured. Since there is a case that carbonmonoxide cannot be measured with the THC meter, it is preferable that aconcentration of carbon monoxide is independently measured with a gasconcentration meter (e.g., commercial name: MEXA-554J produced byHoriba, Ltd.) capable of measuring carbon monoxide.

In a production method of the present invention, at least one conditionselected from the group consisting of temperature rise speed, an amountof air introduced into a burner, an amount of fuel gas introduced into aburner, and an amount of the ceramic formed body arranged in thecombustion furnace is adjusted according to a total concentration of thegas to be measured (that is, in the case of selecting two or more kindsof inflammable gas as the gas to be measured, the sum of theconcentrations). A concentration of inflammable gas in a furnace can bereduced by lowering temperature rise speed, increasing an amount of airintroduced into a burner, reducing an amount of fuel gas introduced intoa burner, and reducing an amount of the ceramic formed body arranged inthe combustion furnace.

In particular, in a production method of the present invention, it ispreferable that feedback control is conducted by adjusting at least onecondition selected from the group consisting of temperature rise speed,an amount of air introduced into a burner, and an amount of fuel gasintroduced into a burner according to a total concentration of the gasto be measured. Among the above four conditions, temperature rise speed,an amount of air introduced into a burner, and an amount of fuel gasintroduced into a burner are adjustable conditions in real time upondegreasing, and feed back control can be conducted in a batch wheredegreasing proceeds. Therefore, for example, even in the case that aconcentration of inflammable gas in a furnace is incidentally raised, itexhibits preferable effect in instantly lower the concentration toeffectively prevent explosion in a furnace.

Further, in a production method of the present invention, the control asdescribed above is conducted so that the total concentration of the gasto be measured is within the range from 0 to 75% of an explosion lowerlimit concentration [L₁]calculated from the following formulae (1) and(2) of the whole gas to be measured. $\begin{matrix}{\lbrack {{Formula}\quad 3} \rbrack\quad{P_{Gn} = {100 \times {C_{Gn}/{\sum\limits_{n = 1}^{n}C_{Gn}}}}}} & \begin{matrix}\quad \\(1)\end{matrix} \\{\lbrack {{Formula}\quad 4} \rbrack\quad{L_{1} = {100/{\sum\limits_{n = 1}^{n}( {P_{Gn}/L_{Gn}} )}}}} & \begin{matrix}\quad \\(2)\end{matrix}\end{matrix}$(where P_(Gn): ratio of a concentration of the gas to be measured to thetotal concentration of the gas to be measured, C_(Gn): concentration(vol. %) of each gas to be measured), L₁: explosion lower limitconcentration (vol. %) of the whole gas to be measured, and L_(Gn):lower limit concentration (vol. %) of each gas to be measured).

The above formula (2) is for calculating the explosion lower limitconcentration of a multi-component system and based on the Le-Chateilerprinciple. Specifically, the explosion lower limit concentration of thewhole gas to be measured is determined by a value obtained bymultiplying, by 100, a reciprocal number of the sum of the values eachobtained by dividing a ratio of each gas to the total concentration ofthe gas to be measured by an explosion lower limit concentration of eachgas to be measured.

As described above, in a production method of the present invention, itis preferable to select a hydrocarbon component and carbon monoxide asgas to be measured. In such a case, the above formulae (1) and (2) canbe transformed into the following lormulae (3) to (5).P _(HC)=100×C _(HC)/(C _(HC) +C _(CO))  (3)P _(CO)=100×C _(CO)/(C _(HC) +C _(CO))  (4)L ₂=100/{(P _(HC) /L _(HC))+(P _(CO) /L _(CO))}  (5)(where P_(HC): ratio of a THC concentration to the total concentrationof THC and carbon monoxide, C_(HC): THC concentration (vol. %), P_(CO):ratio of carbon monoxide concentration to the total concentration of THCand carbon monoxide, C_(CO): carbon monoxide concentration (vol. %), L₂:explosion lower limit concentration (vol. %) of the whole THC and carbonmonoxide, L_(HC): explosion lower limit concentration (vol. %) of aspecific hydrocarbon; and L_(CO): explosion lower limit concentration(vol. %) of carbon monoxide).

Incidentally, it is general that a THC meter on the market expresses avalue of the total concentration of hydrocarbon-based components interms of a concentration of a specific hydrocarbon. Therefore, for“explosion lower limit concentration of a specific hydrocarbon” in theabove formula (3), there may be substituted an explosion lowerconcentration of 5.0 (vol. %) of methane in the case that the THC meterexpresses a concentration of the whole hydrocarbon-based components interms of methane, an explosion lower concentration of 2.12 (vol. %) ofpropane in the case of expressing a value in terms of propane, and anexplosion lower concentration of 2.75 (vol. %) of ethylene in the caseof expressing a value in terms of ethylene.

In a production method of a present invention, it is necessary tocontrol the total concentration of gas to be measured in the degreasingprocess. It is particularly preferable that it is conducted in atemperature range from 150 to 400° C., where an organic type additive isdecomposed/combusted. This is because, since decomposition/combustion ofthe organic type additive is insufficiently started in a temperaturerange below 150° C., and since decomposition/combustion of the organictype additive has almost finished in a temperature range above 400° C.,both are lacking in an actual profit.

In addition, in a production method of the present invention, it isnecessary to conduct the control as described above so that the totalconcentration of the gas to be measured is within the range from 0 to75% of an explosion lower limit concentration [L₁] of the whole gas tobe measured, more preferably from 0 to 25%. A concentration above 75% ofan explosion lower limit concentration [L₁] is not preferable becauseexplosion in a furnace cannot be prevented effectively. Control of aconcentration within the range from 0 to 25% of an explosion lower limitconcentration [L₁] is preferable because explosion in a furnace caneffectively be prevented even in the case that gas to be measured isdistributed uneven in a furnace to locally raise a concentration of thegas to be measured. In the case that it is difficult to completelyremove gas to be measured, it is preferable to control a concentrationwithin the range from 1 to 75% of an explosion lower limit concentration[L₁] of the whole gas to be measured, more preferably 1 to 55%, andparticularly preferably 1 to 25%.

Incidentally, in a production method of the present invention, an oxygengas concentration upon degreasing is preferably controlled to be 12 to22 vol. %. When it is below 12 vol. %, it is not preferable because itmakes decomposition/combustion of an organic type additive slow andfiring time long. When it is above 22 vol. %, it is not preferablebecause there is a fear of increasing a danger of explosion in a furnaceraising a probability of causing a crack in the formed body upon firing.However, it is seldom that an oxygen gas concentration in a furnace iswithout the range of 12 to 22 vol. % unless an oxygen concentration isintentionally lowered when inert gas is introduced, and there are fewcases that an oxygen gas concentration becomes a problem.

In a production method of the present invention, for example, acombustion furnace 10 having a constitution as shown in a schematicsectional view of FIG. 1 can be used. The combustion furnace 10 isprovided with an inner room 4 serving as a space for heating a body tobe heated 2 and a gas burner 8 for generating a combustion flame 6 byspraying and combusting fuel gas. In addition, the combustion furnace 10is provided with setters 12 disposed so that it divides the inner room 4horizontally and constituted so that efficient degreasing/firing can beconducted with a large number of bodies 2 to be heated are arranged onmany stages.

[3] Firing Step

In a production method of the present invention, firing is conductedafter the above degreasing step. Firing is an operation for ensuringpredetermined strength by densifying ceramic raw material by sintering.In a production method of the present invention, firing may be conductedaccording to a conventionally known firing method. Firing conditions(temperature and time) differ depending on a ceramic seed of the ceramicraw material, and suitable conditions are selected according to theceramic seed. For example, in the case of using cordierite forming rawmaterial as an aggregate raw material particle, firing is preferablyconducted at 1410 to 1440° C. for 3 to 12 hours. When the firingconditions (temperature and time) are below the above ranges, sinteringof a cordierite particle is prone to be insufficient, which is notpreferable. On the other hand, when the conditions are above theaforementioned ranges, formed cordierite is prone to be melted, which isnot preferable.

EXAMPLES

A production method of the present invention will hereinbelow bedescribed more specifically with Examples. However, these Examples justshow a part of embodiments of a production method of the presentinvention, and a production method of the present invention should notbe limited on the Examples described below.

Example 1

Production of a ceramic structure was tried by degreasing and firing aceramic formed body in a combustion furnace in an oxygen containingatmosphere.

There was used a ceramic formed body having a honeycomb structure havingan outer diameter of 300 mm, a length of 300 mm, a cell shape of asquare of about 1.8 mm×1.8 mm, a partition wall thickness of 0.3 mm, anda cell density of about 30 cells/cm².

The above ceramic formed body was obtained by mixing 8 parts by weightof methyl cellulose as an organic binder, 20 parts by mass of acommercial poly(methyl methacrylate) resin as a pore forming agent, and0.1 parts by mass of fatty acid soap (potassium laurate) as a dispersantwith respect to 100 parts by mass of ceramic raw material containingkaolin, talc, aluminum hydroxide, alumina, and silica at the ratio of18.5:40:15:14:12.5 to give a mixture, which was kneaded to obtain clay,which was subjected to extrusion forming and further dried by air.

As the combustion furnace for degreasing and firing, there was used asingle kiln 20 having a constitution as shown in schematic sectionalview of FIG. 2. The single kiln 20 was provided with an inner room 4(effective capacity of 8 m²) serving as a space for heating a body to beheated 2 and a gas burner 8 for generating a combustion flame 6 byspraying and combusting fuel gas. In addition, the combustion furnace 20is provided with setters 12 disposed so that it divides the inner room 4horizontally and constituted so that efficient degreasing/firing can beconducted with a large number of bodies 2 to be heated are arranged onmany stages. Further, on a side wall of the inner room 4 was disposed asampling tube 14 made of alumina for communicating the outer space withthe inner room 4 to give a constitution by which sampling of anatmosphere in the inner room 4 can be conducted.

Sixty ceramic formed bodies as described above was arranged in the abovesingle kiln, and degreasing was conducted at 150 to 400° C. attemperature rise speed of 4° C./hr. As fuel gas for a gas burner wasused LNG (liquefied natural gas). Total amount of the organic typeadditive contained in 60 ceramic formed body was 150 kg.

In Example 1, a hydrocarbon based compound and carbon monoxide wasselected as gas to be measured. Therefore, upon degreasing, a THCconcentration and a carbon monoxide concentration in a combustionfurnace were measured, and the total concentration of THC and carbonmonoxide was controlled to be a concentration of 0 to 75% of anexplosion lower limit concentration of the whole THC and carbon monoxideby adjusting an amount of air introduced into a burner according to thetotal concentration. After that, firing was conduced at 1410 to 1430° C.for 10 hours to obtain a ceramic formed body having a honeycomb shape.The results are shown in Table 1.

Incidentally, a THC concentration was measured with a THC meter(Commercial name: EHF-770 produced by Anatec Yanaco Corporation). TheTHC meter expresses a concentration of the whole hydrocarbon basedcompound as a value in terms of propane. In addition, a carbon monoxideconcentration was measured with a gas concentration meter (Commercialname: MEXA-554 by Horiba, Ltd.). The gas concentration meter can measurealso an oxygen gas concentration besides a carbon monoxideconcentration. Therefore, an oxygen gas concentration was measured alsowith this gas concentration meter. The oxygen gas concentration upondegreasing was 16 to 18 vol. %.

The explosion lower limit concentration of the total THC and carbonmonoxide was calculated on the basis of the following formulae (3) to(5). At this time, regarding a ratio [P_(HC)] of THC with respect to thetotal concentration of THC and carbon monoxide and a ratio of a carbonmonoxide concentration with respect to the total concentration of THCand carbon monoxide, “maximum THC concentration,” “maximum COconcentration,” and “maximum THC+CO concentration” in Table 1 wereregarded as a THC concentration [C_(HC)], an carbon monoxideconcentration [C_(CO)], and the total concentration [C_(HC)]+[C_(CO)] ofTHC and carbon monoxide, respectively. With regard to an explosion lowerlimit concentration [L_(HC)] of a specific hydrocarbon, an explosionlower limit concentration of 2.12 (vol. %) of propane was employed sincethe THC meter used in the present example shows a concentration in termsof propane. An explosion lower limit concentration [L_(CO)] of carbonmonoxide was determined as 12.50 (vol. %).P _(HC)=100×C _(HC)/(C _(HC) +C _(CO))  (3)P _(CO)=100×C _(CO)/(C _(HC) +C _(CO))  (4)L ₂=100/{(P _(HC) /L _(HC))+(P _(CO) /L _(CO))}  (5)

(where P_(HC): ratio of a THC concentration to the total concentrationof THC and carbon monoxide, C_(HC): THC concentration (vol. %), P_(CO):ratio of carbon monoxide concentration to the total concentration of THCand carbon monoxide, C_(CO): carbon monoxide concentration (vol. %), L₂:explosion lower limit concentration (vol. %) of the whole THC and carbonmonoxide, L_(HC): explosion lower limit concentration (vol. %) of aspecific hydrocarbon; and L_(CO): explosion lower limit concentration(vol. %) of carbon monoxide) TABLE 1 THC + CO explosion Mass of Amountof Amount of Temperature Maximum Maximum lower limit organic air fuelgas rise speed THC Maximum CO THC + CO concentration matter introducedintroduced 150-400° C. concentration concentration concentration (A)(B/A) (kg) (Nm³/hr) (Nm³/hr) (° C./hr) (vol. %) (vol. %) (vol. %) (vol.%) (%) Example 1 150 190 0.07 4 0.75 0.20 0.95 2.57 37 Comp. Ex. 1 300190 0.7 4 1.58 0.42 2.00 2.57 78 Example 2 300 390 14 4 0.74 0.19 0.932.55 36 Example 3 300 770 2.5 4 0.35 0.09 0.44 2.55 17 Example 4 300 4201.6 8 1.07 0.27 1.34 2.55 53 Example 5 150 780 2.2 0.5 0.02 0.005 0.0252.54 1(Evaluation)

As a result of the above control, the maximum value of the totalconcentration of the whole THC and carbon monoxide upon degreasing couldbe controlled to be 0.95 vol. %. That is, it could be controlled to be aconcentration of 0 to 75% of 2.57 vol. %, which is an explosion lowerlimit concentration of the whole THC and carbon monoxide, and therebyexplosion in the furnace could effectively prevented.

Comparative Example 1

Production of ceramic structures were tried in the same conditionsexcept that the number of formed bodies arranged in the furnace was 120and that the total amount of an organic type additive contained in theformed bodies increased to 300 kg. That is, in Comparative Example 1, anamount of air introduced into a burner, an amount of fuel gas introducedinto a burner, and temperature rise speed at 150 to 400° C. werecompletely the same as those in Example 1. The results are shown inTable 1.

Examples 2 to 4

In Examples 2 and 3, production of ceramic structures was tried in thesame conditions as those in Comparative Example 1 except that amounts ofair and fuel gas introduced was adjusted with maintaining temperaturerise speed of 150 to 400° C. to control the total concentration of THCand carbon monoxide. In Example 4, production of a ceramic structure wastried in the same conditions as those in Comparative Example 1 exceptthat temperature rise speed was adjusted according to amounts of air andfuel gas introduced to control the total concentration of THC and carbonmonoxide. The results are shown in Table 1.

(Evaluation)

Though the conditions in Comparative Example 1 were completely the sameas those in Example 1, the maximum amount of the total concentration ofTHC and carbon monoxide rose up to 2.00 vol. % due to a doubled amountof an organic type additive. This concentration exceeds 75% of 2.57 vol.%, which is the explosion lower limit concentration of the whole of THCand carbon monoxide and is charged with danger of an explosion in thefurnace. In contrast, in each of Examples 2 to 4, the maximum amount ofthe total concentration of THC and carbon monoxide was reduced to 75% orless of the explosion lower limit concentration of the whole of THC andcarbon monoxide, and an explosion in the furnace could effectively beprevented.

In particular, in Example 3, the maximum amount of the totalconcentration of THC and carbon monoxide could be reduced to 25% or lessof 2.55 vol. %, which is the explosion lower limit concentration of thewhole of THC and carbon monoxide, and there was exhibited a preferableeffect of effectively preventing an explosion in the furnace even in thecase that THC and carbon monoxide were distributed uneven in the furnaceto raise a concentration of these kinds of inflammable gas locally.

In addition, in Example 4, temperature rise speed can be made twice asthat of Example 2 with maintaining the maximum value of the totalconcentration of THC and carbon monoxide to be 0 to 75% of 2.55 vol. %,which is the explosion lower limit concentration of the whole of THC andcarbon monoxide, and there was exhibited a preferable very high effectin reducing a production cost and improving production efficiency inaddition to the effect in effectively preventing explosion in thefurnace.

INDUSTRIAL APPLICABILITY

A method for producing a ceramic structure of the present invention cansuitably be used as a method for producing a ceramic structure used invarious usage, such as structural material and electric/electronicmaterial. In particular, it can suitably used for producing a ceramichoneycomb structure having high porosity, which has to use a largeamount of an organic type additive and a ceramic honeycomb structurehaving thin partition walls.

1. A method for producing a ceramic structure by conducting degreasingand firing a ceramic formed body containing a ceramic raw material andan organic type additive in a combustion furnace in an oxygen-containingatmosphere; wherein a concentration [C_(Gn)] of at least one kind ofinflammable gas (gas to be measured) in the combustion furnace ismeasured upon degreasing; and at least one condition selected from thegroup consisting of temperature rise speed, an amount of air introducedinto a burner, an amount of fuel gas introduced into a burner, and anamount of the ceramic formed body arranged in the combustion furnace isadjusted according to a total concentration of the gas to be measured tocontrol the total concentration of the gas to be measured to be aconcentration of 0 to 75% of an explosion lower limit concentration [L₁]calculated from the following formulae (1) and (2) of the whole gas tobe measured. $\begin{matrix}{\lbrack {{Formula}\quad 1} \rbrack\quad{P_{Gn} = {100 \times {C_{Gn}/{\sum\limits_{n = 1}^{n}C_{Gn}}}}}} & \begin{matrix}\quad \\(1)\end{matrix} \\{\lbrack {{Formula}\quad 2} \rbrack\quad{L_{1} = {100/{\sum\limits_{n = 1}^{n}( {P_{Gn}/L_{Gn}} )}}}} & \begin{matrix}\quad \\(2)\end{matrix}\end{matrix}$ (where P_(Gn): ratio of a concentration of the gas to bemeasured to the total concentration of the gas to be measured, C_(Gn):concentration (vol. %) of each gas to be measured), L₁: explosion lowerlimit concentration (vol. %) of the whole gas to be measured, andL_(Gn): lower limit concentration (vol. %) of each gas to be measured)2. A method for producing a ceramic structure according to claim 1,wherein a THC(total hydrocarbon) concentration [C_(HC)] in terms of aspecific hydrocarbon and a carbon monoxide concentration [C_(CO)] in thecombustion furnace is measured upon degreasing; and the totalconcentration [C_(HC)+C_(CO)] of THC and carbon monoxide is adjusted tobe a concentration of 0 to 75% of an explosion lower limit concentration[L₂] calculated from the following formulae (1) and (2) of the explosionlower limit concentration [L₂] of the whole THC and carbon monoxide.P _(HC)=100×C _(HC)/(C _(HC) +C _(CO))  (3)P _(CO)=100×C _(CO)/(C _(HC) +C _(CO))  (4)L ₂=100/{(P _(HC) /L _(HC))+(P _(CO) /L _(CO))}  (5) (where P_(HC):ratio of a THC concentration to the total concentration of THC andcarbon monoxide, C_(HC): THC concentration (vol. %), P_(CO): ratio ofcarbon monoxide concentration to the total concentration of THC andcarbon monoxide, C_(CO): carbon monoxide concentration (vol. %), L₂:explosion lower limit concentration (vol. %) of the whole THC and carbonmonoxide, L_(HC): explosion lower limit concentration (vol. %) of aspecific hydrocarbon; and L_(CO): explosion lower limit concentration(vol. %) of carbon monoxide)
 3. A method for producing a ceramicstructure according to claim 1, wherein feedback control is conducted byadjusting at least one condition selected from the group consisting oftemperature rise speed, an amount of air introduced into a burner, andan amount of fuel gas introduced into a burner according to a totalconcentration of the gas to be measured upon degreasing.
 4. A method forproducing a ceramic structure according to claim 1, wherein the totalconcentration of the gas to be measured is controlled in a temperaturerange from 150 to 400° C.
 5. A method for producing a ceramic structureaccording to claim 1, wherein the total concentration of the gas to bemeasured is controlled to be a concentration of 0 to 25% of theexplosion lower limit concentration [L₁] of the whole gas to be measuredin a temperature range from 150 to 400° C.